This invention relates to a sensor for measuring properties of many kinds of fluid by the socalled electrical heating measurement method.
The term "fluid" as used in the present specification means all kinds of fluid including a gaseous substance, a liquid substance or a solid substance such as powder or of two or more of these substances as well as a fluid of the type whose phase changes with time.
The inventors of this application have already proposed, in Japanese Patent Laid-open Application No. 62(1987)-56849, a sensor as follows.
A sensor for measurement by electrical heating has a core rod covered with an electrically insulating member, a thin metal wire wound about the core, and an electrically insulating member covering the thin metal wire.
In the above-mentioned traditional sensor, the thin metal wire is several times longer than the sensor since the thin metal wire is wound around the core rod. With this, the sensor has some advantageous such that the electric resistance increases according to the length of wire and therefore a large heat flux perunit length of the sensor can be obtained by a small electric current. Further, the sensor in itself is not easily cut, broken or bent.
On the other hand, since the thin metal wire is would around the core rod and spirally configured, a stress-strain occurs in the thin metal wire. And so there is a disadvantage such that, when an annealing treatment is performed in order to prevent the stressstrain thereof, the electric resistance thereof is largely changed from the initial value to some value which varies significantly with time.
the change of electric resistance of each sensor is different from other sensors and so it is difficult to determine the specific value common to each sensor.
Therefore, it is difficult to produce a sensor having a desirable electric resistance both before and after annealing. thus, the sensors are not interchangeable since each sensor has different electric resistance and so a relational equation between the electric resistance and properties of materials must be determined with each sensor.